fix(terra-draw): fallback to default styling when style callbacks in select mode return undefined

[KWangechi]

Description of Changes

• Added nullish coalescing (?? defaultValue) to the callback branch in getStylingValue() so that style functions returning undefined or null correctly fall back to Terra Draw default styles instead of propagating null to the adapter

• Added tests for all geometry types verifying that style callbacks returning undefined default to Terra Draw styles

I am using MapLibre as the mapping library, but since this affects the main package, it should cut across all other mapping libraries and their corresponding adapters.

Link to Issue

Issue #831

PR Checklist

• The PR title follows the conventional commit standard
• There is a associated GitHub issue
• If I have added significant code changes, there are relevant tests
• If there are behaviour changes these are documented